Various instruments such as an engine cylinder, a container of a waterproof watch, a gas appliance and so forth are required to be completely free from a gas or liquid leak or to suppress the leakage within a specified limit. To meet this requirement, such instruments or parts are checked for leakage in their manufacturing process.
As leakage inspection equipments for such a leakage test, pressure variation detecting type leakage inspection equipments have already been put to practical use. The leakage inspection equipments of this type can be roughly divided into two types of inspection systems, one of which applies a positive or negative fluid pressure to the interior of an article under inspection and detects whether a change in the pressure is within a prescribed range, thereby judging whether the article under inspection is non-defective or defective, and the other of which applies a positive or negative fluid pressure to each of the articles under inspection and a comparison tank and measures a variation in the differential pressure therebetween, thereby judging whether the article under inspection is non-defective or defective.
In either system, a fluid pressure is applied to the article under inspection, a variation in the pressure or differential pressure is monitored for a certain period of time after the fluid pressure reached a predetermined value, and the article being inspected is judged non-defective or defective depending upon whether the variation in the pressure or differential pressure is within a prescribed range. In such an inspection, air is usually employed as the fluid. In the case of using pneumatic pressure as the fluid pressure, an error may be introduced into the measured pressure value or differential pressure value owing to various factors such as the temperature of the article under inspection, ambient temperature, humidity, water content adhering to the article under inspection and its slight deformation by pressure.
If the error value is always constant, no particular trouble occurs since it is necessary only to hold the reference value for judgement constant. Since the error contained in the pressure or differential pressure value is caused by the abovesaid various factors, however, a change in each factor incurs a variation in the error value. Accordingly, in the case of continuously performing the inspection, it is necessary that the reference value for judgement be frequently modified in response to variations in the error value.
As a method for automatically changing the reference value in response to variations in the error value, the present applicant has previously proposed pressure variation detecting type leakage inspection equipment (PCT/JP 83/00085) in applicant's prior copending U.S. application Ser. No. 674,908.
The pressure variation detecting type leakage inspection equipment proposed in said U.S. application Ser. No. 674,908 is provided with a memory for storing a predetermined number of measured data on inspected articles judged non-defective, calculating means for calculating a mean value of the measured data stored in the memory and data correcting means for correcting measured data through using the mean value, obtained by the calculating means, as a correction value for the measured data on the article being inspected, and is adapted so that by utilizing a moving mean of a plurality of preceding measured data as a correction value, the tendency of variation in an error value resulting from various factors is detected, ensuring automatic correction of variations in the error value.
With this previously proposed pressure variation detecting type leakage inspection equipment, even if the error value varies owing to various factors, the correction value is automatically corrected in response to the variation in the error value. As a result of this, an appropriate correction value can always be obtained and the leakage inspection equipment can be automated.